Recently, with the rise in demand for large and slim displays, plasma display panels (hereinafter abbreviated as “PDP”) are becoming widely popular.
Since near-infrared light emitted from a PDP causes electronic equipment that use near-infrared remote controls to malfunction, a filter using a near-infrared light absorbing dye to block infrared rays is necessary.
Also, near-infrared light blocking filters are widely used in optical lenses, glass for automobiles and buildings, and the like.
In these applications, the near-infrared light blocking filter is required to effectively absorb light in the near-infrared spectrum, while allowing the transmission of visible light, and to possess high resistance to heat and moisture.
Various near-infrared light blocking filters comprising a diimmonium salt compound have been proposed (Japanese Patent Application Laid-open No. 10-180922).
This publication discloses various near-infrared light absorbing dyes of diimmonium salt. Of these, N,N,N′,N′-tetrakis{p-di(n-butyl)aminophenyl}-p-phenylene diimmonium salt which comprises bis(hexafluoroantimonate) as the anion moiety is commonly used due to the relatively excellent resistance to heat and moisture.
However, resistance to heat and moisture of this near-infrared light absorbing dye is still inadequate. Specifically, the dye possesses some problems. For example the dye is decomposed during use thereby decreasing absorbance of near-infrared light. An aminium salt generated from decomposition of the dye causes visible light to be absorbed thereby decreasing transmissivity of visible light and resulting in yellow coloration which degrades color tone.
Furthermore, since this dye comprises a heavy metal as an anion moiety, use of a large quantity of this dye results in environmental pollution.
A first object of the present invention is to provide a novel near-infrared light absorbing dye with excellent resistance to heat and moisture, wherein near-infrared light absorbance does not decrease when used for a long period of time. A second object of the present invention is to provide a near-infrared light blocking filter possessing excellent resistance to heat and moisture.